Recently, a rechargeable secondary battery has been widely used as an energy source for wireless mobile devices.
In addition, the secondary battery has been prominent as a power source of an electric vehicle (EV), a hybrid electric vehicle (HEV), etc., that have been suggested as a scheme for solving air pollution of an existing gasoline vehicle, diesel vehicle, etc., using a fossil fuel.
Small-sized mobile devices use one or more battery cells per one device. In contrast, due to necessity of high output and large capacity, middle and large-sized devices such as a vehicle, etc., use a middle and large-sized battery pack in which a plurality of battery cells are electrically interconnected.
In this case, it is preferable to manufacture the middle and large-sized battery pack at the size and weight as small as possible. Therefore, a square type battery, a pouch type battery, or the like, that can be highly integrated and have a small weight compared with capacity have been mainly used as a battery cell of the middle and large-sized battery pack. Among those, the pouch type battery having a small weight, an electrolyte that is less likely to be leaked and inexpensive manufacturing costs has been largely interested.
As a unit battery (battery cell) of a middle and large-sized battery pack, a nickel-hydrogen secondary battery has been mainly used. Recently, similar to the small battery pack, a lithium secondary battery providing high output compared with capacity has been largely researched and a portion thereof has reached commercialization.
However, the lithium secondary battery basically has low safety. In particular, the pouch type battery is a prominent candidate as a unit battery of the middle and large-sized battery pack due to various advantages as described above, but has low mechanical rigidity and a great risk of fire due to a leakage of pyrophoric materials such as an electrolyte, or the like, when a sealing part is separated.
In addition, in the pouch type secondary battery, since an electrolyte is injected into cells all of which are integrated, when each cell is overcharged, voltage increases and an electrolyte is decomposed in the cells due to overheating to generate combustible gas in the cells and increase pressure in the pouch, which leads to a swelling phenomenon that swells the pouch. Further, a cathode and an anode are short-circuited due to melting of a separator between the cathode and the anode, which causes ignition. Therefore, it is possible to secure safety of a battery.
In actual, a safety test, such as overcharging, forced discharging, or the like, and various safety tests, such as a high temperature storage test, a thermal shock test, a thermal exposure test, or the like, are performed before a completed battery is distributed in the market due to a problem of safety of a lithium secondary battery. The thermal safety test is performed by leaving a battery for several tens of minutes to several tens of hours under various temperature conditions. In this case, the battery should not be exploded and ignited. In the worst case, the sealing of the battery is released, thereby preventing the battery from being exploded and ignited.
As described above, in the pouch type secondary battery, an excessive amount of gas is generated due to overcurrent. As a result, a technology of removing a problem in which pressure in a battery is increased has been disclosed from the past.
Korean Patent No. 0560158 (“lithium secondary batter, hereinafter, referred to as related art 1) disclosed a structure of an explosion protection safety apparatus in which an outer surface of a package of a battery is partially formed with an adhesive layer to increase an internal pressure of a battery to a threshold or more and instantly rupture a package when the package is expanded, thereby preventing the battery from being exploded.
As described above, the related art 1 may solve a problem to some degree in that it is possible to prevent a cell from being suddenly exploded. As a result, since the exhaust starts when the package of a portion at which an adhesive layer is not formed is ruptured during the expansion of the cell, the strength in explosion of cell can be reduced to some degree but the cells are still likely to be exploded. Therefore, the related art 1 cannot completely solve the problem.
Therefore, at the time of the occurrence of the swelling phenomenon of the lithium secondary battery, a development of various technologies for preventing the lithium secondary battery from being exploded or ignited is needed.